The present invention relates to a method and apparatus for modifying the transverse cross section of a body such as a rod, disk or cylinder and, in a preferred embodiment, relates to a method and apparatus for modifying the cross section of refractory rods, such as optical fiber preforms.
While potentially useful in a wide variety of applications, the present invention evolved in the field of optical fiber fabrication. Optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. Optical fibers are typically manufactured by constructing an optical fiber preform of appropriate composition and drawing fiber from the preform. A typical preform is in the form of a concentric glass rod having a length of about one meter and an outer diameter of about 20 mm. The inner core of the rod is a high purity, low loss glass such as germanium silicate glass having a diameter of about 1-2 mm. The concentric outer cylinder, referred to as cladding, provides protection to the inner core. The cladding is a layer of glass with a lower index of refraction than the inner core.
In the manufacture of optical fibers from preforms, it is important that the center of the preform core coincide with the center of the preform cross section. In the event of eccentricity in the preform, the fiber drawn from the preform will also be eccentric. As a result, when one eccentric optical fiber is connected to another optical fiber, the transmitting cores will not coincide, and there will be appreciable light transmission loss due to misalignment of the cores.
To prevent the production of eccentric fiber, the preforms are carefully inspected before the drawing process. Preforms which do not meet stringent concentricity requirements are discarded. The consequence is a relatively high rejection rate of expensive preforms. Accordingly, a method for modifying the transverse cross sections of optical preforms so as to reduce preform eccentricity would be highly advantageous in the production of optical fibers.